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   <h1>Creating disk images for gem5</h1>
   <time>Nov 24, 2015 • Jason Lowe-Power</time>
   <hr>
   <p>When using gem5 in full-system mode, you have to have a disk image with
 the operating system and all of your data on it. This is just like
 having a physical disk in a physical machine. In this post, I’m going to
 walk through how to create a new disk and install a (semi-)current
 version of Ubuntu on the disk. By the end of this post, you should be
 able to create your own disk with whatever extra data and applications
 you want.</p>

 <p>This post assumes that you have already checked out a version of gem5
 and can build and run gem5 in full-system mode. The <a href="http://www.lowepower.com/jason/learning_gem5/">Learning
 gem5</a> documentation is a
 good place to start. This post uses the x86 ISA for gem5, and is mostly
 applicable to other ISAs. More details on setting up ARM systems can be
 found on the gem5 wiki:
 <a href="http://gem5.org/Ubuntu_Disk_Image_for_ARM_Full_System">http://gem5.org/Ubuntu_Disk_Image_for_ARM_Full_System</a>.</p>

 <p>In the future, this post may be folded into <a href="http://www.lowepower.com/jason/learning_gem5/">Learning
 gem5</a>.</p>

 <h2 id="creating-a-blank-disk-image">Creating a blank disk image</h2>

 <p>The first step is to create a blank disk image (usually a .img file).
 Luckily, the gem5 developers have already made this easy with a tool
 that is simple to use. To create a blank disk image, which is formatted
 with ext2 by default, simply run the following.</p>

 <div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>&gt; util/gem5img.py init ubuntu-14.04.img 4096
 </code></pre></div></div>

 <p>This command creates a new image, called “ubuntu-14.04.img” that is 4096
 MB. This command may require you to enter the sudo password, if you
 don’t have permission to create loopback devices. <em>You should never run
 commands as the root user that you don’t understand! You should look at
 the file util/gem5img.py and ensure that it isn’t going to do anything
 malicious to your computer!</em></p>

 <p>We will be using util/gem5img.py heavily throughout this post, so you
 may want to understand it better. If you just run <code class="highlighter-rouge">util/gem5img.py</code>, it
 displays all of the possible commands.</p>

 <div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>Usage: %s [command] &lt;command arguments&gt;
 where [command] is one of
     init: Create an image with an empty file system.
     mount: Mount the first partition in the disk image.
     umount: Unmount the first partition in the disk image.
     new: File creation part of "init".
     partition: Partition part of "init".
     format: Formatting part of "init".
 Watch for orphaned loopback devices and delete them with
 losetup -d. Mounted images will belong to root, so you may need
 to use sudo to modify their contents
 </code></pre></div></div>

 <h2 id="copying-root-files-to-the-disk">Copying root files to the disk</h2>

 <p>Now that we have created a blank disk, we need to populate it with all
 of the OS files. Ubuntu distributes a set of files explicitly for this
 purpose. You can find the <a href="https://wiki.ubuntu.com/Core">Ubuntu core</a>
 distribution for 14.04 at
 <a href="http://cdimage.ubuntu.com/ubuntu-core/releases/14.04/release/">http://cdimage.ubuntu.com/ubuntu-core/releases/14.04/release/</a> Since I
 am simulating an x86 machine, I chose the file
 <code class="highlighter-rouge">ubuntu-core-14.04-core-amd64.tar.gz</code>. Download whatever image is
 appropriate for the system you are simulating.</p>

 <p>Next, we need to mount the blank disk and copy all of the files onto the
 disk.</p>

 <div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>mkdir mnt
 ../../util/gem5img.py mount ubuntu-14.04.img mnt
 wget http://cdimage.ubuntu.com/ubuntu-core/releases/14.04/release/ubuntu-core-14.04-core-amd64.tar.gz
 sudo tar xzvf ubuntu-core-14.04-core-amd64.tar.gz -C mnt
 </code></pre></div></div>

 <p>The next step is to copy a few required files from your working system
 onto the disk so we can chroot into the new disk. We need to copy
 <code class="highlighter-rouge">/etc/resolv.conf</code> onto the new disk.</p>

 <div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>sudo cp /etc/resolv.conf mnt/etc/
 </code></pre></div></div>

 <h2 id="setting-up-gem5-specific-files">Setting up gem5-specific files</h2>

 <h3 id="create-a-serial-terminal">Create a serial terminal</h3>

 <p>By default, gem5 uses the serial port to allow communication from the
 host system to the simulated system. To use this, we need to create a
 serial tty. Since Ubuntu uses upstart to control the init process, we
 need to add a file to /etc/init which will initialize our terminal.
 Also, in this file, we will add some code to detect if there was a
 script passed to the simulated system. If there is a script, we will
 execute the script instead of creating a terminal.</p>

 <p>Put the following code into a file called /etc/init/tty-gem5.conf</p>

 <div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code># ttyS0 - getty
 #
 # This service maintains a getty on ttyS0 from the point the system is
 # started until it is shut down again, unless there is a script passed to gem5.
 # If there is a script, the script is executed then simulation is stopped.

 start on stopped rc RUNLEVEL=[12345]
 stop on runlevel [!12345]

 console owner
 respawn
 script
    # Create the serial tty if it doesn't already exist
    if [ ! -c /dev/ttyS0 ]
    then
       mknod /dev/ttyS0 -m 660 /dev/ttyS0 c 4 64
    fi

    # Try to read in the script from the host system
    /sbin/m5 readfile &gt; /tmp/script
    chmod 755 /tmp/script
    if [ -s /tmp/script ]
    then
       # If there is a script, execute the script and then exit the simulation
       exec su root -c '/tmp/script' # gives script full privileges as root user in multi-user mode
       /sbin/m5 exit
    else
       # If there is no script, login the root user and drop to a console
       # Use m5term to connect to this console
       exec /sbin/getty --autologin root -8 38400 ttyS0
    fi
 end script
 </code></pre></div></div>

 <h3 id="setup-localhost">Setup localhost</h3>

 <p>We also need to set up the localhost loopback device if we are going to
 use any applications that use it. To do this, we need to add the
 following to the <code class="highlighter-rouge">/etc/hosts</code> file.</p>

 <div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>127.0.0.1 localhost
 ::1 localhost ip6-localhost ip6-loopback
 fe00::0 ip6-localnet
 ff00::0 ip6-mcastprefix
 ff02::1 ip6-allnodes
 ff02::2 ip6-allrouters
 ff02::3 ip6-allhosts
 </code></pre></div></div>

 <h3 id="update-fstab">Update fstab</h3>

 <p>Next, we need to create an entry in <code class="highlighter-rouge">/etc/fstab</code> for each partition we
 want to be able to access from the simulated system. Only one partition
 is absolutely required (<code class="highlighter-rouge">/</code>); however, you may want to add additional
 partitions, like a swap partition.</p>

 <p>The following should appear in the file <code class="highlighter-rouge">/etc/fstab</code>.</p>

 <div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code># /etc/fstab: static file system information.
 #
 # Use 'blkid' to print the universally unique identifier for a
 # device; this may be used with UUID= as a more robust way to name devices
 # that works even if disks are added and removed. See fstab(5).
 #
 # &lt;file system&gt;    &lt;mount point&gt;   &lt;type&gt;  &lt;options&gt;   &lt;dump&gt;  &lt;pass&gt;
 /dev/hda1      /       ext3        noatime     0 1
 </code></pre></div></div>

 <h3 id="copy-the-m5-binary-to-the-disk">Copy the <code class="highlighter-rouge">m5</code> binary to the disk</h3>

 <p>gem5 comes with an extra binary application that executes
 pseudo-instructions to allow the simulated system to interact with the
 host system. To build this binary, run <code class="highlighter-rouge">make -f Makefile.&lt;isa&gt;</code> in the
 <code class="highlighter-rouge">gem5/m5</code> directory, where <code class="highlighter-rouge">&lt;isa&gt;</code> is the ISA that you are simulating
 (e.g., x86). After this, you should have an <code class="highlighter-rouge">m5</code> binary file. Copy this
 file to /sbin on your newly created disk.</p>

 <p>After updating the disk with all of the gem5-specific files, unless you
 are going on to add more applications or copying additional files,
 unmount the disk image.</p>

 <div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>&gt; util/gem5img.py umount mnt
 </code></pre></div></div>

 <h2 id="install-new-applications">Install new applications</h2>

 <p>The easiest way to install new applications on to your disk, is to use
 <code class="highlighter-rouge">chroot</code>. This program logically changes the root directory (“/”) to a
 different directory, mnt in this case. Before you can change the root,
 you first have to set up the special directories in your new root. To do
 this, we use <code class="highlighter-rouge">mount -o bind</code>.</p>

 <div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>&gt; sudo /bin/mount -o bind /sys mnt/sys
 &gt; sudo /bin/mount -o bind /dev mnt/dev
 &gt; sudo /bin/mount -o bind /proc mnt/proc
 </code></pre></div></div>

 <p>After binding those directories, you can now <code class="highlighter-rouge">chroot</code>:</p>

 <div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>&gt; sudo /usr/sbin/chroot mnt /bin/bash
 </code></pre></div></div>

 <p>At this point you will see a root prompt and you will be in the <code class="highlighter-rouge">/</code>
 directory of your new disk.</p>

 <p>You should update your repository information.</p>

 <div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>&gt; apt-get update
 </code></pre></div></div>

 <p>You may want to add the universe repositories to your list with the
 following commands. Note: The first command is require in 14.04.</p>

 <div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>&gt; apt-get install software-properties-common
 &gt; add-apt-repository universe
 &gt; apt-get update
 </code></pre></div></div>

 <p>Now, you are able to install any applications you could install on a
 native Ubuntu machine via <code class="highlighter-rouge">apt-get</code>.</p>

 <p>Remember, after you exit you need to unmount all of the directories we
 used bind on.</p>

 <div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>&gt; sudo /bin/umount mnt/sys
 &gt; sudo /bin/umount mnt/proc
 &gt; sudo /bin/umount mnt/dev
 </code></pre></div></div>

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	<h1>Creating disk images for gem5</h1>
	<time>Nov 24, 2015 • Jason Lowe-Power</time>
	<hr>
	<p>When using gem5 in full-system mode, you have to have a disk image with
	the operating system and all of your data on it. This is just like
	having a physical disk in a physical machine. In this post, I’m going to
	walk through how to create a new disk and install a (semi-)current
	version of Ubuntu on the disk. By the end of this post, you should be
	able to create your own disk with whatever extra data and applications
	you want.</p>

	<p>This post assumes that you have already checked out a version of gem5
	and can build and run gem5 in full-system mode. The <a href="http://www.lowepower.com/jason/learning_gem5/">Learning
	gem5</a> documentation is a
	good place to start. This post uses the x86 ISA for gem5, and is mostly
	applicable to other ISAs. More details on setting up ARM systems can be
	found on the gem5 wiki:
	<a href="http://gem5.org/Ubuntu_Disk_Image_for_ARM_Full_System">http://gem5.org/Ubuntu_Disk_Image_for_ARM_Full_System</a>.</p>

	<p>In the future, this post may be folded into <a href="http://www.lowepower.com/jason/learning_gem5/">Learning
	gem5</a>.</p>

	<h2 id="creating-a-blank-disk-image">Creating a blank disk image</h2>

	<p>The first step is to create a blank disk image (usually a .img file).
	Luckily, the gem5 developers have already made this easy with a tool
	that is simple to use. To create a blank disk image, which is formatted
	with ext2 by default, simply run the following.</p>

	<div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>> util/gem5img.py init ubuntu-14.04.img 4096
	</code></pre></div></div>

	<p>This command creates a new image, called “ubuntu-14.04.img” that is 4096
	MB. This command may require you to enter the sudo password, if you
	don’t have permission to create loopback devices. <em>You should never run
	commands as the root user that you don’t understand! You should look at
	the file util/gem5img.py and ensure that it isn’t going to do anything
	malicious to your computer!</em></p>

	<p>We will be using util/gem5img.py heavily throughout this post, so you
	may want to understand it better. If you just run <code class="highlighter-rouge">util/gem5img.py</code>, it
	displays all of the possible commands.</p>

	<div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>Usage: %s [command] <command arguments>
	where [command] is one of
	init: Create an image with an empty file system.
	mount: Mount the first partition in the disk image.
	umount: Unmount the first partition in the disk image.
	new: File creation part of "init".
	partition: Partition part of "init".
	format: Formatting part of "init".
	Watch for orphaned loopback devices and delete them with
	losetup -d. Mounted images will belong to root, so you may need
	to use sudo to modify their contents
	</code></pre></div></div>

	<h2 id="copying-root-files-to-the-disk">Copying root files to the disk</h2>

	<p>Now that we have created a blank disk, we need to populate it with all
	of the OS files. Ubuntu distributes a set of files explicitly for this
	purpose. You can find the <a href="https://wiki.ubuntu.com/Core">Ubuntu core</a>
	distribution for 14.04 at
	<a href="http://cdimage.ubuntu.com/ubuntu-core/releases/14.04/release/">http://cdimage.ubuntu.com/ubuntu-core/releases/14.04/release/</a> Since I
	am simulating an x86 machine, I chose the file
	<code class="highlighter-rouge">ubuntu-core-14.04-core-amd64.tar.gz</code>. Download whatever image is
	appropriate for the system you are simulating.</p>

	<p>Next, we need to mount the blank disk and copy all of the files onto the
	disk.</p>

	<div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>mkdir mnt
	../../util/gem5img.py mount ubuntu-14.04.img mnt
	wget http://cdimage.ubuntu.com/ubuntu-core/releases/14.04/release/ubuntu-core-14.04-core-amd64.tar.gz
	sudo tar xzvf ubuntu-core-14.04-core-amd64.tar.gz -C mnt
	</code></pre></div></div>

	<p>The next step is to copy a few required files from your working system
	onto the disk so we can chroot into the new disk. We need to copy
	<code class="highlighter-rouge">/etc/resolv.conf</code> onto the new disk.</p>

	<div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>sudo cp /etc/resolv.conf mnt/etc/
	</code></pre></div></div>

	<h2 id="setting-up-gem5-specific-files">Setting up gem5-specific files</h2>

	<h3 id="create-a-serial-terminal">Create a serial terminal</h3>

	<p>By default, gem5 uses the serial port to allow communication from the
	host system to the simulated system. To use this, we need to create a
	serial tty. Since Ubuntu uses upstart to control the init process, we
	need to add a file to /etc/init which will initialize our terminal.
	Also, in this file, we will add some code to detect if there was a
	script passed to the simulated system. If there is a script, we will
	execute the script instead of creating a terminal.</p>

	<p>Put the following code into a file called /etc/init/tty-gem5.conf</p>

	<div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code># ttyS0 - getty
	#
	# This service maintains a getty on ttyS0 from the point the system is
	# started until it is shut down again, unless there is a script passed to gem5.
	# If there is a script, the script is executed then simulation is stopped.

	start on stopped rc RUNLEVEL=[12345]
	stop on runlevel [!12345]

	console owner
	respawn
	script
	# Create the serial tty if it doesn't already exist
	if [ ! -c /dev/ttyS0 ]
	then
	mknod /dev/ttyS0 -m 660 /dev/ttyS0 c 4 64
	fi

	# Try to read in the script from the host system
	/sbin/m5 readfile > /tmp/script
	chmod 755 /tmp/script
	if [ -s /tmp/script ]
	then
	# If there is a script, execute the script and then exit the simulation
	exec su root -c '/tmp/script' # gives script full privileges as root user in multi-user mode
	/sbin/m5 exit
	else
	# If there is no script, login the root user and drop to a console
	# Use m5term to connect to this console
	exec /sbin/getty --autologin root -8 38400 ttyS0
	fi
	end script
	</code></pre></div></div>

	<h3 id="setup-localhost">Setup localhost</h3>

	<p>We also need to set up the localhost loopback device if we are going to
	use any applications that use it. To do this, we need to add the
	following to the <code class="highlighter-rouge">/etc/hosts</code> file.</p>

	<div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>127.0.0.1 localhost
	::1 localhost ip6-localhost ip6-loopback
	fe00::0 ip6-localnet
	ff00::0 ip6-mcastprefix
	ff02::1 ip6-allnodes
	ff02::2 ip6-allrouters
	ff02::3 ip6-allhosts
	</code></pre></div></div>

	<h3 id="update-fstab">Update fstab</h3>

	<p>Next, we need to create an entry in <code class="highlighter-rouge">/etc/fstab</code> for each partition we
	want to be able to access from the simulated system. Only one partition
	is absolutely required (<code class="highlighter-rouge">/</code>); however, you may want to add additional
	partitions, like a swap partition.</p>

	<p>The following should appear in the file <code class="highlighter-rouge">/etc/fstab</code>.</p>

	<div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code># /etc/fstab: static file system information.
	#
	# Use 'blkid' to print the universally unique identifier for a
	# device; this may be used with UUID= as a more robust way to name devices
	# that works even if disks are added and removed. See fstab(5).
	#
	# <file system> <mount point> <type> <options> <dump> <pass>
	/dev/hda1 / ext3 noatime 0 1
	</code></pre></div></div>

	<h3 id="copy-the-m5-binary-to-the-disk">Copy the <code class="highlighter-rouge">m5</code> binary to the disk</h3>

	<p>gem5 comes with an extra binary application that executes
	pseudo-instructions to allow the simulated system to interact with the
	host system. To build this binary, run <code class="highlighter-rouge">make -f Makefile.<isa></code> in the
	<code class="highlighter-rouge">gem5/m5</code> directory, where <code class="highlighter-rouge"><isa></code> is the ISA that you are simulating
	(e.g., x86). After this, you should have an <code class="highlighter-rouge">m5</code> binary file. Copy this
	file to /sbin on your newly created disk.</p>

	<p>After updating the disk with all of the gem5-specific files, unless you
	are going on to add more applications or copying additional files,
	unmount the disk image.</p>

	<div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>> util/gem5img.py umount mnt
	</code></pre></div></div>

	<h2 id="install-new-applications">Install new applications</h2>

	<p>The easiest way to install new applications on to your disk, is to use
	<code class="highlighter-rouge">chroot</code>. This program logically changes the root directory (“/”) to a
	different directory, mnt in this case. Before you can change the root,
	you first have to set up the special directories in your new root. To do
	this, we use <code class="highlighter-rouge">mount -o bind</code>.</p>

	<div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>> sudo /bin/mount -o bind /sys mnt/sys
	> sudo /bin/mount -o bind /dev mnt/dev
	> sudo /bin/mount -o bind /proc mnt/proc
	</code></pre></div></div>

	<p>After binding those directories, you can now <code class="highlighter-rouge">chroot</code>:</p>

	<div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>> sudo /usr/sbin/chroot mnt /bin/bash
	</code></pre></div></div>

	<p>At this point you will see a root prompt and you will be in the <code class="highlighter-rouge">/</code>
	directory of your new disk.</p>

	<p>You should update your repository information.</p>

	<div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>> apt-get update
	</code></pre></div></div>

	<p>You may want to add the universe repositories to your list with the
	following commands. Note: The first command is require in 14.04.</p>

	<div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>> apt-get install software-properties-common
	> add-apt-repository universe
	> apt-get update
	</code></pre></div></div>

	<p>Now, you are able to install any applications you could install on a
	native Ubuntu machine via <code class="highlighter-rouge">apt-get</code>.</p>

	<p>Remember, after you exit you need to unmount all of the directories we
	used bind on.</p>

	<div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>> sudo /bin/umount mnt/sys
	> sudo /bin/umount mnt/proc
	> sudo /bin/umount mnt/dev
	</code></pre></div></div>

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